The present invention relates to a ventilation system for dynamoelectric machines, and more particularly to cooling the stators of large machines such as hydro generators.
Hydro or waterwheel generators are vertical machines of large physical size with salient pole rotors, and rotate at relatively low speeds as compared to other types of generators although the peripheral speed of the rotor may be quite high because of the large diameter. These machines are cooled by circulation of air and two types of cooling systems have conventionally been used. In one system, the air is circulated by a blower, which may be mounted on the rotor, and is directed to flow axially through the spaces between the salient poles of the rotor. The air flows axially in the interpolar spaces to cool the field windings and poles and then flows radially into the air gap and across the air gap to the stator. The stator core has radial vent ducts extending through it and the cooling air crossing the air gap flows through these ducts to cool the stator windings and core, and is discharged at the back or outer periphery of the core and recirculated after flowing through the coolers. The other cooling system, which has come into use more recently, uses the rotor spider and rim as a blower to cause the air to flow radially through ducts in the rotor rim and thus into the interpolar spaces from which it flows radially across the air gap and through the stator core radial vent ducts as described above. In both of these schemes, therefore, the air rotates with the rotor as it flows into and across the air gap. Furthermore, the same air flows in series through the rotor and the stator core, and the volume of air must be adequate to cool both rotor and stator sufficiently to keep the temperature rise within the required limits. Thus, a large volume of air is necessary and the rotation of this large mass of air at or near the peripheral speed of the rotor results in high windage losses.
The present trend in hydro machines, especially in those intended for alternative operation as generators and as motors in pumped storage installations, is to machines of large size and relatively high speed. The combination of large rotor diameter and high speed results in very high peripheral velocities of the rotor which may be in excess of 15,000 feet per minute, for example. With the conventional ventilation schemes discussed above, the windage loss is quite high and becomes a relatively large percentage of the total loss in the machine. A reduction in the windage loss therefore can result in a very substantial reduction in the total losses, with a corresponding increase in efficiency, or a reduction in the size of the machine with a substantial saving in cost.
It has been proposed to greatly reduce the windage loss in machines of this type by separating the rotor and stator air flows into separate ventilation systems and sealing off the rotor air flow from the air gap, as in Kilgore et al. U.S. Pat. No. 3,588,557. This results in a substantial reduction in the windage loss since most of the rotor cooling air is confined to the rotor and prevented from reaching the air gap as only enough air is allowed to flow into the air gap to remove the heat resulting from the pole face losses. Separation of the rotor and stator air flows also greatly reduces the volume of air required. In the conventional cooling schemes, the stator cooling air flows first through the rotor and then across the air gap so that it is heated by the rotor losses before it reaches the stator. In order to obtain the necessary cooling of the stator, therefore, a relatively large flow of air is required. When the stator air flow is separated from the rotor air flow, however, cold air enters the stator so that the volume of air required for cooling the stator is reduced and the total air flow in the machine is greatly reduced. Since the stator air flow is largely independent of the rotor air flow, and smaller volumes of cooler air can be used for cooling the stator, an opportunity exists for further improvement in cooling by new stator ventilation systems.
In a copending application of A. K. Mishra et al, Ser. No. 587,790, filed July 16, 1975 and assigned to the assignee of the present invention, there is disclosed and claimed an improved stator ventilation system. In this system, cooling air is introduced through the back, or outside, of the stator core and flows inward through radial vent ducts in the core to the region of the teeth and windings. The air then flows axially in longitudinal ducts in the core from one vent duct to the next and then radially outward through the core. Such a cooling system results in a substantial improvement in efficiency of the machine by reducing the pumping power required to circulate the stator cooling air as well as permitting a higher machine rating or smaller size because of the improved cooling.